The present invention relates to an apparatus for driving and measuring a MEMS mirror system, the MEMS mirror system having a mirror pivotable around an axis by a driving coil and exhibiting a resonance frequency. The invention further relates to a method for driving and measuring said MEMS mirror system.
Mirror systems in the form of micro-electro-mechanical systems (MEMS) are known in the art as devices capable of pivoting a mirror around an axis at a high frequency, e.g. in order to deflect a laser beam in a specific direction. This can be used for projectors which project light along a viewing zone, e.g. with a rate of a few thousand cycles per second.
One type of state of the art MEMS mirror systems, e.g. as disclosed in EP 1 515 424 A2, drives the mirror by oscillating it around an axis at a resonance frequency, yielding a sinusoidal oscillation of the mirror. In such MEMS mirror systems the mirror is driven by feeding a pulse train with the resonance frequency into a coil attached to the mirror and lying in a magnetic field. At times when there is no pulse fed into the system, the counter-voltage induced by the movement of the coil in the magnetic field can be measured and used to tune the pulse frequency and phase in order to maintain a fixed amplitude of the mirror movement based on the measured amplitude and phase of the induced signal. However, the movement of the resonating mirror is restricted to a sinusoidal pattern of a varying, non-linear angular velocity, making it unsuitable for a series of applications. Furthermore, the mirror can be driven at the resonance frequency of the MEMS mirror system only. Since this resonance frequency is subject to the physical properties of the system, it cannot be changed readily.
Other types of state of the art MEMS mirror systems, as disclosed in, e.g. Arda D. Yalcinkaya et al., “Two-Axis Electromagnetic Microscanner for High Resolution Displays”, IEEE Journal of Microelectromechanical Systems, Vol. 15, No. 4, August 2006, pp. 786-794, drive the mirror at a constant angular velocity by feeding a triangular or sawtooth signal with a frequency well below the resonance frequency to the coil to avoid the mirror accidentally changing into a sinusoidal oscillating state. With this type of driving system, a movement of the mirror can only be determined by observing the actual optical deflection of the laser beam, e.g. on a test screen. This necessitates an external physical test setup and is particularly complex when a whole array of MEMS mirror systems, e.g. as part of a display, is to be observed.